Termination and Contact Structures for a High Voltage Gan-Based Heterojunction Transistor

ABSTRACT

A semiconductor device is provided that includes a substrate, a first active layer disposed over the substrate, and a second active layer disposed on the first active layer. The second active layer has a higher bandgap than the first active layer such that a two-dimensional electron gas layer arises between the first active layer and the second active layer. A termination layer, which is disposed on the second active layer, includes InGaN. Source, gate and drain contacts are disposed on the termination layer.

RELATED APPLICATIONS

This application is a continuation of application Ser. No. 13/066,200filed Apr. 7, 2011, which is a continuation of application Ser. No.11/725,823 filed Mar. 20, 2007, which applications are assigned to theassignee of the present application. This application is related tocopending U.S. patent application Ser. No. 11/725,760, now U.S. Pat. No.7,501,670, entitled “Cascode Circuit Employing A Depletion-Mode,GaN-Based Fet,” filed on Mar. 20, 2007 and herein incorporated byreference in its entirety herein.

This application is also related to copending U.S. patent applicationSer. No. 11/725,820, entitled “High-Voltage GaN-Based HeterojunctionTransistor Structure and Method of Forming Same,” filed on Mar. 20, 2007and herein incorporated by reference in its entirety herein.

FIELD OF THE INVENTION

The present invention relates to a high voltage transistorheterostructure, and more particularly relates to a high voltage galliumnitride (GaN) high electron mobility transistor (HEMT).

BACKGROUND OF THE INVENTION

Gallium nitride (GaN) offers substantial opportunity to enhanceperformance of electronic devices such as high electron mobilitytransistors (HEMTs). The HEMT behaves much like a conventional FieldEffect Transistor (FET), and the fabrication of HEMT devices is based onFET architecture. However, HEMTs require a very precise, lattice-matchedheterojunction between two compound semiconductor layers. In general, aGaN HEMT has a Schottky layer and a GaN buffer layer deposited on asubstrate and source, gate, and drain contacts deposited on the Schottkylayer.

The GaN-based HEMT device is capable of maximizing electron mobility byforming a quantum well at the heterojunction interface between the AlGaNlayer, which has a large band gap, and the GaN layer, which has anarrower band gap. As a result, electrons are trapped in the quantumwell. The trapped electrons are represented by a two-dimensionalelectron gas in the undoped GaN layer. The amount of current iscontrolled by applying voltage to the gate electrode, which is inSchottky contact with the semiconductors so that electrons flow alongthe channel between the source electrode and the drain electrode.

As the market for HEMTS continues to grow, many improvements remaindesirable to enhance various operating characteristics such as thebreakdown voltage Vbr and the leakage current I. For example, oneproblem that remains to be adequately addressed arises because theSchottky layer is typically metallic and may be exposed to air duringfabrication of the HEMT and/or during operation of the HEMT. By exposingthe Schottky layer to air, surface reactions such as oxidation may occuron the surface of the Schottky layer. These surface reactions maydegrade the performance of the HEMT and also decrease the effectivenessof passivation. Passivation is the deposition of a dielectric materialon the surface of the HEMT in order to passivate, or fill, surface trapson the surface of the HEMT, thereby avoiding device degradation due tothese surface traps such as RF to DC dispersion.

Therefore, there remains a need for a high voltage GaN HEMT structurethat, among other things, has a reproducible termination layer capableof preventing surface reactions during fabrication and operation of theGaN HEMT.

SUMMARY OF THE INVENTION

In accordance with the present invention, a semiconductor device isprovided that includes a substrate, a first active layer disposed overthe substrate, and a second active layer disposed on the first activelayer. The second active layer has a higher bandgap than the firstactive layer such that a two-dimensional electron gas layer arisesbetween the first active layer and the second active layer. Atermination layer, which is disposed on the second active layer,includes InGaN. Source, gate and drain contacts are disposed on thetermination layer.

In accordance with one aspect of the invention, the first active layercomprises a group III nitride semiconductor material. the first activelayer comprises GaN.

In accordance with another aspect of the invention, the second activelayer comprises a group III nitride semiconductor material.

In accordance with another aspect of the invention, the second activelayer comprises Al_(X)Ga_(1-X)N, wherein 0<X<1.

In accordance with another aspect of the invention, the second activelayer is selected from the group consisting of AlGaN, AlInN, andAlInGaN.

In accordance with another aspect of the invention, a nucleation layeris disposed between the substrate and the first active layer.

In accordance with another aspect of the invention, a semiconductordevice includes a substrate, a first active layer disposed over thesubstrate, and a second active layer disposed on the first active layer.The second active layer has a higher bandgap than the first active layersuch that a two-dimensional electron gas layer arises between the firstactive layer and the second active layer. A termination layer isdisposed on the second active layer. The termination layer is selectedfrom the group consisting of Fe-doped GaN, Si-doped GaN, FeN and SiN.Source, gate and drain contacts are disposed on the termination layer.

In accordance with another aspect of the invention, a semiconductordevice includes a substrate, a first active layer disposed over thesubstrate, and a second active layer disposed on the first active layer.The second active layer has a higher bandgap than the first active layersuch that a two-dimensional electron gas layer arises between the firstactive layer and the second active layer. The second active layerincludes first and second recesses formed therein. A source and draincontact are disposed in the first and second recesses, respectively. Agate electrode is disposed over the second active layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows one embodiment of a gallium nitride (GaN) heterojunctionstructure incorporated in a high electron mobility transistor (HEMT).

FIGS. 2 and 3 show alternative embodiments of a gallium nitride (GaN)heterojunction structure incorporated in a high electron mobilitytransistor (HEMT).

FIG. 4 shows another alternative embodiment of a gallium nitrideheterojunction structure incorporated in a high electron mobilitytransistor (HEMT).

DETAILED DESCRIPTION

It is worthy to note that any reference herein to “one embodiment” or“an embodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the invention. The appearances of thephrase “in one embodiment” in various places in the specification arenot necessarily all referring to the same embodiment Moreover, thevarious embodiments may be combined in a multiplicity of ways to yieldadditional embodiments that are not explicitly shown herein.

The present invention relates to a high voltage, gallium nitride (GaN)heterojunction structure incorporated in a high electron mobilitytransistor (HEMT) 10 as illustrated in FIG. 1. The HEMT 10 includes asubstrate 12, a nucleation (transitional) layer 18, a GaN buffer layer22, an aluminum gallium nitride (Al_(X)Ga_(1-X)N; 0<X<1) Schottky layer24, and a cap or termination layer 16. Further, the HEMT 10 includessource contact 27, gate contact 28, and drain contact 30.

The GaN heterojunction structure 10 is typically fabricated using anepitaxial growth process. For instance, a reactive sputtering processmay be used where the metallic constituents of the semiconductor, suchas gallium, aluminum and/or indium, are dislodged from a metallic targetdisposed in close proximity to the substrate while both the target andthe substrate are in a gaseous atmosphere that includes nitrogen and oneor more dopants. Alternatively, metal organic chemical vapor deposition(MOCVD) may be employed, wherein the substrate is exposed to anatmosphere containing organic compounds of the metals as well as to areactive nitrogen-containing gas, such as ammonia, and adopant-containing gas while the substrate is maintained at an elevatedtemperature, typically around 700-1100 C. The gaseous compoundsdecompose and form a doped semiconductor in the form of a film ofcrystalline material on the surface of the substrate 302. The substrateand the grown film are then cooled. As a further alternative, otherepitaxial growth methods, such as molecular beam epitaxy (MBE) or atomiclayer epitaxy may be used. Yet additional techniques that may beemployed include, without limitation, Flow Modulation OrganometallicVapor Phase Epitaxy (FM-OMVPE), Organometallic Vapor-Phase Epitaxy(OMVPE), Hydride Vapor-Phase Epitaxy (HVPE), and Physical VaporDeposition (PVD).

To begin the growth of the structure, the nucleation layer 18 isdeposited on the substrate 12. The substrate 12 may be formed fromvarious materials including, but not limited to, sapphire or siliconcarbide (SiC). The nucleation layer 18 may be, for example, an aluminumrich layer such as Al_(X)Ga_(1-X)N, where X is in the range 0 to 1. Thenucleation layer 18 operates to correct a lattice mismatch between theGaN buffer layer 22 and the substrate 12. In general, a lattice mismatchis created when the spacing between atoms of one layer does not matchthe spacing between the atoms of an adjacent layer. As a result of thelattice mismatch, bonding between the atoms of the adjacent layers areweak, and the adjacent layers could crack, separate, or have a largenumber. of crystalline defects. Therefore, the nucleation layer 18operates to correct the lattice mismatch between the GaN buffer layer 22and the substrate 12 by creating an interface between the crystallinestructure of the substrate 12 and the crystalline structure of the GaNbuffer layer 22.

After depositing the nucleation layer 18, the GaN buffer layer 22 isdeposited on the nucleation layer 18, and the Al_(X)Ga_(1-X)N Schottkylayer 24 is deposited on the GaN buffer layer 22. The two-dimensionalconduction channel 26, which is a thin, high mobility channel, confinescarriers to an interface region between the GaN buffer layer 22 and theAl_(X)Ga_(1-X)N Schottky layer 24. The cap or termination layer 16 isdeposited on the Al_(X)Ga_(1-X)N Schottky layer 24 and serves to protectthe Al_(X)Ga_(1-X)N Schottky layer 24 from surface reactions, such asoxidation, during fabrication and operation of the HEMT 10. Because theSchottky layer 24 includes aluminum, oxidation occurs if theAl_(X)Ga_(1-X)N Schottky layer 24 is exposed to air and is not otherwiseprotected.

After growth of the epitaxial layers 18, 22 and 24 and the terminationlayer 16 on the substrate 12, the HEMT 10 is completed by depositing thesource, gate, and drain contacts 27, 28, and 30, respectively, on thetermination layer 16. Each of the contacts 27, 28, and 30 are metalliccontacts. Preferably, the gate contact 28 is a metallic material such asbut not limited to nickel, gold, and the source and drain contacts 27and 30, are each a metallic material such as but not limited totitanium, gold, or aluminum.

In one embodiment of the invention the termination layer 16 is an InGaNlayer that is formed on the Al_(X)Ga_(1-X)N Schottky layer 24. The InGaNlayer 16 serves two purposes, the first of which is to provide an upperlayer that does not include Al so that oxidation is reduced. Moreover,by using an InGaN material instead of a material that includes Al, thegrowth process may be simplified since Al-containing compounds such asInGaAlN generally require higher growth temperatures to provide adequateuniformity and smoothness. In addition, the InGaN layer 24 slightlylowers the potential barrier at the surface, which can reduce the buildup of surface charges and reduce the leakage current on the surface ofthe structure.

In another embodiment of the invention the termination layer 16 is aflash layer comprising Al metal. A flash layer is formed with a veryshort burst of material. This will form a very thin (e.g., 1-2monolayers of material) but even coverage over the structure's surface.The flash layer is generally performed in situ. To ensure that metallicAl is formed and not AlN, the reactive nitrogen-containing gas (e.g.,ammonia) that would otherwise be present when forming AlN is absent. TheAl flash layer may be formed at high or low temperatures. After itsformation, the Al can be subsequently annealed to form a thin oxidelayer. Since the Al flash layer is very thin, it can be oxidized in itsentirety, thus creating an initial “native” oxide on the material whichthen protects the Schottky layer 24 from undergoing any degradation ofthe type that is often seen in processing. This can also act as anadditional barrier material for reduction of leakage currents andincrease in breakdown voltage, both of which are important to HEMTperformance. Instead of Al, the flash layer may comprise other metalssuch as gallium or even indium. The Ga or In flash layer can also beoxidized to form a uniform “native” oxide on the structure.

In yet other embodiments of the invention the cap or termination layer16 may be formed from other materials such as highly Fe doped GaN, Sidoped GaN, FeN or SiN. These layers, which may be epitaxial,nonepitaxial or even amorphous, can serve as initial passivation layersor as additional barrier materials to reduce leakage currents andincrease breakdown voltages. For instance, the addition of Fe to GaNresults in a material that can reduce the leakage current because thematerial is more insulating and reduces electron mobility.

In other embodiments of the invention, a thin AlN layer may be formed onthe Al_(X)Ga_(1-X)N Schottky layer 24. This layer provides an additionalSchottky barrier layer to help modulate the charge more efficiently,thus reducing the leakage current and increasing the breakdown voltageof the device. The AlN layer may also serve as an initial passivationlayer for the structure, since the AlN can be easily wet etched todeposit ohmic contacts. Alternatively, the AlN layer may be oxidized toform a passivation layer.

In some embodiments, the termination layer 16 is approximately 1 to 5nanometers thick. Therefore, electrons can easily tunnel through thetermination layer 16. As a result, the termination layer 16 does notincrease the Schottky barrier height between the gate contact 28 and theAl_(X)Ga_(1-X)N Schottky layer 24, where the Schottky barrier heightdefines a potential energy barrier encountered by electrons at theinterface of the gate contact 28 and the Al_(X)Ga_(1-X)N Schottky layer24. Further, the termination layer 16 does not affect the formation ofthe source and drain contacts 27 and 30.

FIG. 2 shows yet another embodiment of the invention in which the ohmiccontacts 27 and 30 are located in recesses formed in the Al_(X)Ga_(1-X)NSchottky layer 24. The recesses are formed by etching theAl_(X)Ga_(1-X)N Schottky layer 24 in accordance with conventionaltechniques. The recesses may extend partially or completely through theAl_(X)Ga_(1-X)N Schottky layer 24. For instance, in some cases therecess may extend to a depth of about 5 to 15 nm deep, thereby allowinga sufficient thickness of the Al_(X)Ga_(1-X)N Schottky layer 24 toremain to create the channel layer 26. By recessing the contacts in thismanner the contact resistivity and the smoothness of the surface isreduced to increase the penetration of the metals deposited to form theohmic contact. The increased surface roughness results in better metalmigration into the semiconductor). For devices requiring lowon-resistance, this arrangement can be significant in achieving thelowest possible on-resistance. As shown in FIG. 4, this embodiment ofthe invention may also employ a cap or termination layer such as thosediscussed above. In this case the recesses in which the contacts 27 and30 are located will also extend through the termination layer.

FIG. 3 shows another embodiment of the invention in which the barrierlayer 24 is formed from AlInGaN instead of Al_(X)Ga_(1-X)N. Forinstance, as discussed in M. Asif Khan et al., “Strain Energy BandEngineering in AlGaInN/GaN Heterostructure Field Effect Transistors,”GAAS99, Al_(x)In_(y)Ga_((1-x-y))N junctions are employed which have abarrier thickness less than 50 nm with alloy compositions that vary fromx equals 0.1 to 0.2 and y equals 0.00 to 0.02). Furthermore, Khan et al.states that an Al/In ratio of 5 should be nearly lattice matched to GaN,based on a linear interpolation of lattice constants. By using AlInGaNthe strain can be controlled independently of the bandgap, therebyallowing the bandgap of the material to be altered with more freedom inregards to critical thickness. For power devices this can be critical toobtain the most charge in the channel without unduly stressing thematerial and reducing device lifetime, which might otherwise occur asthe material relaxes over time.

Although various embodiments are specifically illustrated and describedherein, it will be appreciated that modifications and variations of thepresent invention are covered by the above teachings and are within thepurview of the appended claims without departing from the spirit andintended scope of the invention. For example, while the depletion modeFET has been described as a GaN-based device, the invention moregenerally encompasses a depletion mode FET that is formed from any GroupIII nitride compound semiconductor in which the group III element may begallium (Ga), aluminum (Al), boron (B) or indium (In).

1-20. (canceled)
 21. A method of fabricating high electron mobilitytransistor (HEMT) comprising: depositing a nucleation layer on asubstrate; forming a first active layer over the nucleation layer;forming a second active layer over the first active layer, the secondactive layer having a higher bandgap than the first active layer suchthat a two-dimensional conduction channel is formed in an interfaceregion between the first active layer and the second active layer;forming a flash layer over the second active layer that passivates asurface of the HEMT; forming a gate contact disposed on the flash layer;and forming a source contact and a drain contact respectively disposedin first and second recesses that extend through the flash layer intothe second active layer.
 22. The method according to claim 21, whereinthe first active layer comprises a group Ill nitride semiconductormaterial.
 23. The method according claim 21, wherein the first activelayer comprises GaN.
 24. The method according to claim 21, wherein thesecond active layer comprises a group III nitride semiconductormaterial.
 25. The method according to claim 24, wherein the secondactive layer comprises Al_(X)Ga_(1-X)N, wherein 0<X<1.
 26. The methodaccording to claim 21, wherein the forming of the flash layer isperformed in situ in a reactive chamber absent of a nitrogen-containinggas.
 27. The method according to claim 26, wherein the flash layercomprises Al without a nitrogen component.
 28. The method according toclaim 21 wherein the flash layer comprises Ga.
 29. The method accordingto claim 21 wherein the flash layer comprises In.
 30. The methodaccording to claim 21 further comprising annealing the flash layer. 31.The method according to claim 21 further comprising oxidizing the flashlayer.